CMake: also verify libzmq.so version

Before, we used the libzmq.so symlink to link to ZMQ. This can
be problematic when users have multiple ZMQ installations because
the symlink can point to an arbitrary version.
Merge pull request #205 from toxeus/zmqLibVersion

CMake: also verify libzmq.so version
Removed leftover debugging assert.
Merge pull request #253 from benma/otcontract

Removed leftover debugging assert.